1. Field of the Invention
The present invention relates to a technique for subjecting a target substrate to a plasma process, and specifically to a plasma processing technique of the type that applies an RF (radio frequency) to an electrode to generate plasma. Particularly, the present invention relates to a plasma processing technique utilized in a semiconductor process for manufacturing semiconductor devices. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices and FPDs, plasma is often used for processes, such as etching, deposition, oxidation, and sputtering, so that process gases can react well at a relatively low temperature. Parallel-plate plasma processing apparatuses of the capacitive coupling type are in the mainstream of plasma processing apparatuses of the single substrate type.
In general, a parallel-plate plasma processing apparatus of the capacitive coupling type includes a process container or reaction chamber configured to reduce the pressure therein, and an upper electrode and a lower electrode disposed therein in parallel with each other. The lower electrode is grounded and configured to support a target substrate (semiconductor wafer, glass substrate, or the like) thereon. The upper electrode and/or lower electrode are supplied with RF voltage through a matching unit. At the same time, a process gas is delivered from a showerhead provided on the upper electrode side. Electrons are accelerated by an electric field formed between the upper electrode and lower electrode and collide with the process gas, thereby ionizing the gas and generating plasma. Neutral radicals and ions derived from the plasma are used to perform a predetermined micro-fabrication on the surface of the substrate. In the process described above, the two electrodes function to form a capacitor.
In recent years, miniaturization proceeds in the design rules used for manufacturing processes, and thus plasma processes are required to generate higher density plasma at a lower pressure. Under the circumstances, there is a trend in parallel-plate plasma processing apparatuses of the capacitive coupling type described above, such that the RF applied to the upper electrode is selected from a range covering higher frequencies (for example, 50 MHz or more) than conventional values (typically, 27 MHz or less). However, if the frequency of the RF applied to the upper electrode is set higher, when the RF is supplied from an RF power supply through a feed rod to the electrode backside, it is transmitted through the electrode surface by means of the skin effect and is concentrated at the central portion of the electrode bottom surface (plasma contact surface). As a consequence, the electric field intensity at the central portion of the electrode bottom surface becomes higher than the electric field intensity at the peripheral portion, so the density of generated plasma becomes higher at the electrode central portion than at the electrode peripheral portion. Further, since plasma radially diffuses from a high density area to a low density area, the distribution of the plasma changes such that the density of the plasma becomes increasingly higher at the electrode central portion than at the electrode peripheral portion.
In order to solve this problem, a design is known in which the bottom surface central portion of an upper electrode is formed of a high resistivity member (for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-323456). According to this technique, the high resistivity member is employed for the bottom surface central portion of an upper electrode to consume more RF power as Joule heat there. As a consequence, the electric field intensity on the bottom surface (plasma contact surface) of the upper electrode is more reduced at the electrode central portion than at the electrode peripheral portion, so that the low uniformity described above in plasma density is remedied.
However, the high resistivity member employed for the bottom surface central portion of an upper electrode may consume too much RF power as Joule heat (energy loss). Further, the influence of inductive reactance components, such as a feed rod used for a feed line to the upper electrode, become more pronounced, if the frequency of RF power is increased. As a consequence, a resonance point may be formed at an inconstant position on the feed line or upper electrode, and an abnormally large electric current thereby flows near the resonance point.
Incidentally, recent plasma processing apparatuses typically include an upper electrode provided with a number of gas through-holes, from which a process gas is delivered toward a lower electrode, i.e., the upper electrode is used as a so-called showerhead as well. In the case of such an upper electrode of the showerhead type, the upper electrode is attacked and sputtered by ions from the plasma, and thus is considered as a consumable part. Particularly, the delivery port (corner) of each gas through-hole tends to be sputtered because of the electric field concentration thereat. If the gas delivery port is scraped, gas supplied therefrom spreads divergently, thereby making it difficult to stably generate plasma. For this reason, the sputtered level (flared level) of gas delivery ports is used as an index for the electrode service life. Accordingly, the service life of an upper electrode of the showerhead type is shortened with increase in plasma density.